The center outer and rignht side inner are the bits for an WM20 box
Apart from that you should replace the layshaft as the B series box should have the speedo drive on the end of it
If you just want to look like it is the right box then just replace the outer cover and pop a plug in the speedo cable hole
You will need this hole to add oil unless you fit a Wm inner as well .
Quite a few people run a B series main case with an M series inner & outer.
This allows them to sue B seried sprockets that are cheaper & easier to come by and of course it also hase a proper oil seal so does not leak through the sleeve gear if you should do a stupid thing like actually ride your bike
It is not so much that I feel an urge to have the bike look like an original, it does not do that anyway, not by a long shot. The gearbox that I have at the moment is a bit troublesome with false neutrals and a very strange gearing. If there is a seal on the drive side it has long ago creased doing its job. As oil is leaking out quite heavily.
Anyways, thanks for the input. Now I know a bit more and I might, or might not move forward with building a gearbox.